1. Field of the Invention
The present invention relates to the processing of video signals, and more particularly to the processing of video signals representing many levels or shades of gray in a black and white picture.
2. Description of the Prior Art
The typical black and white picture is made up of 48 perceivable shades of gray covering the full dynamic range of what the eye normally sees. In a primarily dark scene with some light area included, there are not enough contrasts of this mainly dark scene to provide sufficient contrast to make the detail visible to the normal eye. In such a scene, too many levels or shades of gray will be allocated to the light shades even though they do not occur very often. In known prior art video processing systems, it is common to take a received video signal and stretch it over a full dynamic range of 48 levels, or shades, to enhance the contrast of the signal, regardless of how many levels of shade the signal actually possesses. This enables objects to be visually detected on a television monitor that previously could not be seen. Such prior art systems were effective for enhancing the contrast between a signal that is either very dark or very light and, thereby not having much contrast. However, where there exists a signal having dark shades, very little moderate shades and some light shades, the known systems provide very little contrast enhancement due to the inordinate amount of time devoted to the enhancement of the signal where there is a small amount of moderate shade. Where such types of signals are being processed, there is a minimum amount of contrast enhancement because there is no discrimination between signals that occur more often as compared with the signals or shades that rarely occur. Also, in such systems, each of the shades of the input signals is allocated the same number of shades of gray as the output. Therefore, the dark shades which have a low signal level but occur more often than the remaining levels of signals are actually not provided with any contrast enhancement.